1. Field of the Invention
The present invention relates to a sheet paper conveying device, an image forming apparatus and a sheet paper conveying method to convey sheet paper in prescribed direction while separating paper one by one.
2. Description of the Related Art
As a device to convey sheet paper in the direction of an image forming apparatus in such image forming apparatus as printers, copying machines or facsimiles, a paper feeding mechanism comprising feed rollers and opposite rollers that are split separately is so far disclosed in Japanese Patent Publication No. 2002-209275. Also, a paper conveying mechanism of which pressure rollers opposing to conveying rollers are divided into plural units each of which is pressed against the conveying rollers is disclosed in Japanese Patent Application No. 2003-182445. However, either of the above-mentioned conventional paper conveying mechanisms have not a function to separate more than 2 sheets of overlapped paper one by one.
On the other hand, there is a sheet paper conveying device provided with paper supply rollers and opposing separation rollers. In this device, while the paper supply rollers are rotated in the paper conveying direction, the separation rollers are rotated in the direction reverse to the sheet conveying direction and only one sheet of paper kept in contact with the paper supply roller is separated and conveyed. In this conventional apparatus that has a separation and conveying function, a pressing load of the separation rollers (hereinafter, referred to as pushing pressure W) against the paper supply roller and a separation torque T of the separation rollers are concerned as factors to determine a set value for separating paper and a separation torque T of the separation roller. This pushing pressure W and the separation torque T are in such a relation that when the separation torque T is kept constant, the separation margin increases and the paper supply margin decreases and the conveyance becomes defective when the pushing pressure W is made small. On the other hand, when the separation torque T is kept constant and the pushing pressure W is increased, the paper supply margin increases, the separation margin decreases and the defective separation is caused.
Because of this antithetical relation of the separation margin and the paper supply margin, to determine the separation torque T and the pushing pressure W of a paper conveying device that has the separation function, it was necessary to measure the separation margin or the paper supply margin by a paper conveying device in an image forming apparatus and set the separation torque T and the pushing pressure W in an area wherein both the separation margin and the paper supply margin can be secured. In other words, when separation torque T, pushing pressure W, coefficient of friction of the paper supply roller μrp and coefficient of margin between paper are μrp>T/W>μpp, both the separation margin and the paper supply margin can be secured (the coefficient of friction referred to here is y/x when a weight of paper that can be conveyed at a fixed load x is y). For example, the separation margin and paper supply margin were so far decided while balancing T/W, by assuming a coefficient of friction of paper supply rollers at 1.3 (130 g paper can be conveyed at 100 g load) and a coefficient of friction between paper at 0.6 (60 g paper can be conveyed at 100 g load).
However, with the diversification of paper and popularization of reclaimed paper, the conveyance of paper having a higher value of coefficient of friction between paper μpp is demanded in recent years. Therefore, the balance adjustment of the separation torque T and the pushing pressure W is difficult and depending on kind of paper, paper cannot be separated or supplied in some cases. Further, when tried to increase a separation margin or paper supply margin for achieving separation and conveyance of various kinds of paper, the service life of the paper supply rollers and separation rollers can be shortened and practical use of them may be impeded.
Accordingly, in the sheet paper conveying device, it is demanded to expand kinds of paper for separation conveyance and to keep the long service life of the paper supply rollers and the separation rollers.